This application claims the priority of Japanese Patent Application No. 2000-003455 filed on Jan. 12, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens disposed at a front end part of an endoscope; and, in particular, to an endoscope objective lens equipped with a focusing mechanism.
2. Description of the Prior Art
In the field of electronic endoscope, there have been known general scopes (routine-only apparatus) suitable for wide-field viewing to which a fixed focus employed for general routine inspections (inspections in a complete physical examination and the like) is applied.
Meanwhile, optical enlargement type electronic endoscopes have recently come into use, which enable wide-field viewing and can observe a specific viewing site (in a field of view which extends about 2 to 5 mm from the viewing site) displayed on a monitor under a magnification of 70xc3x97 to 100xc3x97, for example, by partly moving the objective optical system at the front end part of the endoscope in the optical axis direction.
In such an optical enlargement type electronic endoscope, due to demands from medical fields in practice, it is also absolutely necessary for an image displayed on a monitor at the time of normal viewing ranging about 5 to 100 mm from the viewing site (i.e., an image at the time of wide-field viewing) to yield the best image quality on a par with that of an image obtained when a general scope (routine-only apparatus) employing a fixed focus is used, whereby optical designing is carried out on the basis of such an idea.
Due to such circumstances, the optical enlargement type electronic endoscope is designed such that the image quality at the time of normal viewing ranging about 5 to 1000 mm from the viewing site is preferentially on a par with that of a general scope, whereby curvature of field remarkably increases at the time of enlarging the object in the optical enlargement type electronic endoscope.
Namely, if a lens is moved in the enlarging direction (field-narrowing direction) in order to carry out viewing under magnification in an optical enlargement type electronic endoscope, then the curvature of field greatly increases at marginal parts of a picture.
When viewing an object having a relatively flat form, a so-called floating mechanism may be used so as to absorb the curvature of field upon close-up (upon enlargement). However, the viewing distance often varies between the center part of picture and its marginal parts in object images to be viewed under magnification by an endoscope, as in protruded objects such as polyps, recessed objects such as ulcers, and so forth. Therefore, it is difficult for normal corrections for curvature of field to become effective. Namely, normal curvature corrections must be carried out while assuming that the object surface is flat, whereby there may occur a situation where marginal parts, in particular, are out of focus depending on the form of viewing site.
In view of the circumstances mentioned above, it is an object of the present invention to provide an endoscope objective lens mounted to an endoscope having an enlarging function and enabling a switch to wide-field viewing, which can appropriately correct the curvature of field occurring according to the form of a viewing site, and can yield an image which is in focus to marginal parts of a picture even at the time of close-up.
The endoscope objective lens in accordance with the present invention comprises at least one movable lens group, the movable lens group being movable so as to generate a curvature of field such that an image quality of a marginal part of a picture becomes favorable according to a form of an object while keeping a substantially constant magnification.
The endoscope objective lens may be an optical system having a variable focus or a fixed focus.
The endoscope objective lens may comprise, successively from the object side, a first lens group having a negative refracting power, a second lens group having a positive refracting power, and a third lens group having a positive refracting power;
the second and third lens group being movable along an optical axis so as to generate the curvature of field.
In this case, the endoscope objective lens may further comprise a stop disposed near a lens constituting the second lens group on the object side.
The endoscope objective lens may comprise, successively from the object side, a first lens group having a negative refracting power, a second lens group having a positive refracting power, a third lens group having a negative refracting power, and a fourth lens group having a positive refracting power;
the second and third lens group being movable along an optical axis so as to generate the curvature of field.
In this case, the endoscope objective lens may further comprise a stop disposed between lenses in the third lens group.